Area Forecast Discussion
Issued by NWS Knoxville/Tri Cities, TN

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FXUS64 KMRX 200822
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
422 AM EDT Sat May 20 2017

.SHORT TERM (Today and Tonight)...
Latest water vapor and RAP upper air analysis indicate broad
shortwave trough in the NW Gulf of Mexico this morning with a
well defined upper level low over NE Colorado. Meanwhile, the
upper level ridge axis extends N from Florida across Michigan.

The Gulf shortwave will ride NE along the western edge of the
ridge today where it looks to merge with an impulse ejecting from
the base of the Colorado upper low as they move into SE TN late
today. Although best dynamics and instability lies west of the
area through today, deep moisture, coupled with modest MLCAPE on
the order of 1000 J/kg and a surface trough lifting NE out of
Alabama and Mississippi late today should help to ignite some late
afternoon and evening convection primarily over the southern
third of the area. This surface trough is then expected to
continue to push NE across the area overnight tonight with showers
and thunderstorms spreading NE into the central and northern
Valley by Sunday morning.

The ECMWF, GFS, and NAM are all indicating a deep layer moisture
surge into the southern valley by this evening which reaches into
the central valley by morning. With PWs reaching above 1.5 inches,
which is above the 90th percentile for this time of year, much of
the central and southern areas could get a fairly decent rainfall
through tonight with 24 hour totals as high as 0.75 inches fcst
with some guidance even higher (SREF Plumes as high as 1.0
inches). Additionally, locally higher totals to over an inch in
thunderstorms across the southern valleys are possible by Sunday
morning. Areas in the northern Valley and SW Virginia will likely
see minimal totals of around 0.25 inches by Sunday morning.

.LONG TERM (Sunday Through Friday)...
Precipitation chances really start to ramp up as we move into
Saturday night/Sunday. As the front approaches the wind fields
start to turn more southerly and increase in speed, bringing in in
more moisture to our area. On Saturday evening/night there is
moderate CAPE available for storms to work with, but not much in
the way of mid/low level shear. So we could see a few storms pulse
up quickly and threaten to produce strong gusty winds or some
hail, but widespread severe weather is not anticipated on Saturday
(nor Sunday). As the front actually gets closer the precipitation
should become more widespread with the best chances for rain all
week coming on Sunday/Sunday night. Will have to monitor
precipitation amounts on Sunday as PWAT values of around 1.5" or
higher, putting us into the 90th percentile for this time of year.
Some places could see several rounds of thunderstorms run over
them leading to localized flooding.

Once we get into the work week things quiet down as the
precipitation will exit off to the east throughout the day and
temperatures are expected to be around normal. There will only be a
brief lull in precipitation before a second system zips through the
area. This one should move through quickly and only bring a round of
rain on Tuesday night/Wednesday. It will also cool temperatures
down, with highs on Thursday struggling to make it to the 70`s.
Quiet weather is then expected through the rest of the work week.


&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             88  69  78  64 /  60  80  80  70
Knoxville McGhee Tyson Airport, TN  88  68  76  61 /  20  70  80  70
Oak Ridge, TN                       89  69  77  61 /  30  60  80  70
Tri Cities Airport, TN              88  64  75  59 /  40  40  70  80

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...None.
TN...None.
VA...None.
&&

$$

EJH/TD


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